The field of the invention relates to X-ray computed tomography (CT) and more particularly to computed tomography of manufactured parts.
In computed tomography a radiographic source (e.g., X-ray) typically is positioned on a first side of an object and a radiographic (e.g., X-ray) detector is positioned on a second, opposing side of the object. X-rays originating from the radiographic source pass through the object and are detected in the X-ray detector. Relative motion is generated between the object and the radiographic X-ray source and X-ray detector, such that radiographic image data is collected at a plurality of relative positions with the object. The radiographic X-ray source and X-ray detector may also be moved relative to the axis of the object as well as around the object. In some industrial applications the X-ray source and X-ray detector are commonly held stationary and the object is rotated and translated to provide the plurality of relative positions. In some other industrial applications the X-ray source and X-ray detector are commonly held stationary and the object is rotated or translated to provide the plurality of relative positions.
Often the radiographic X-ray detector is made up of a number of X-ray detectors aligned in a two-dimensional array. The use of a two-dimensional array of X-ray detectors allows for a separate two-dimensional radiographic image to be collected at each step of rotation and translation of the object relative to the source and the detector. Alternatively, the use of a two-dimensional array of X-ray detectors allows for a separate two-dimensional radiographic image to be collected at each step of rotation or translation of the object relative to the source and the detector.
The intensity of X-rays passing through an object is related to the integral of the object”s linear attenuation coefficient along the path of the X-ray. Where the object contains defects or other non-uniformities, the flux density of the X-rays passing through the object will vary with the physical linear attenuation coefficient of the non-uniformity.
A number of digital radiographs are collected during rotation and translation of the object relative to the X-ray detector and radiographic X-ray source. Alternatively, a number of digital radiographs are collected during rotation or translation of the object relative to the X-ray detector and radiographic X-ray source. The digital radiographic data from these images is processed and a three-dimensional image is reconstructed to facilitate location of defects and non-uniformities in a three-dimensional representation of the object. One approach for image reconstruction involves solving a matrix of equations; a value of the linear attenuation coefficient for each point may be determined in three-dimensional space using the so-called Algebraic Reconstruction Technique (ART). Alternatively, a filtered backprojection technique may be used where digital radiographic data is preprocessed, filtered, and then backprojected into three-dimensional space to generate a three-dimensional reconstruction of the linear attenuation coefficients within the object.
With knowledge of each point”s linear attenuation coefficient in three-dimensional space, images are created from such linear attenuation coefficients. The object is figuratively sliced (e.g., a plane may be formed through the object) and an image of the slice is created using the linear attenuation coefficient at each point on the slice as pixel values of the image.
While tomographic images can be useful, the images may have artifacts resulting from the configuration of the imaging geometry. For example, X-ray flux that traverses the object is composed of two components: a primary X-ray signal and a scattered X-ray signal. The primary X-ray signal results from X-rays that do not interact with the object. The scattered X-ray signal results from X-rays that interact with the object and are redirected (i.e., scattered).
Some of the scattered X-rays may be directed away from the X-ray detector, or the surrounding material may absorb the scattered X-rays. Other scattered X-rays may reach and be detected in the X-ray detector at a variety of angles. In X-ray computed tomography, detection of only the primary X-ray signal is desired. The scattered X-ray signal is known to reduce resolution and contrast in reconstructed images.
The conventional solution to reduce the scattered X-ray signal involves the use of physical collimators. However, physical collimators are commonly most effective in one-dimensional (linear) X-ray detectors where the X-ray detector elements are relatively large, such that stationary collimators are feasible. Multi-row X-ray detectors also use physical collimators; the dimensions of individual X-ray detector elements are still relatively large to make collimation of the primary X-ray signal feasible.
Where area X-ray detectors (i.e., two-dimensional X-ray detectors) are used, the physical dimension of the individual X-ray detector elements may be an order of magnitude less than those used in linear X-ray detectors. Hence, it is not typically feasible to collimate every X-ray detector element of the area X-ray detector, as is usually done with linear or multi-row X-ray detectors. As a result, the scattered X-ray component detected by the area X-ray detector is significantly larger than the scattered X-ray component detected by the linear X-ray detector. Therefore, a need exists for a means for reducing the scattered X-ray signal.